Arc discharge lamps, particularly of the metal halide variety, are frequently employed in commercial usage because of their high luminous efficacy and long life. A typical metal halide lamp includes a quartz or fused silica arc tube that is hermetically sealed within a borosilicate glass outer envelope. The arc tube, itself hermetically sealed, has tungsten electrodes sealed into opposite ends and contains an arc generating and sustaining medium. This medium, or fill, includes mercury, metal halide additives which generally include the halides of sodium, cesium and scandium, and a rare gas to facilitate starting. Starting aids, in the form of auxiliary electrodes, as is well known in the art, or ultraviolet sources, as shown in U.S. Pat. Nos. 4,818, 915 and 5,323,091, can also be used. In high wattage lamps the outer envelope is filled with nitrogen or other inert gas at less than atmospheric pressure. In low wattage lamps, such as those operated at about 100 watts, the outer envelope is evacuated and provided with gettering material to maintain the vacuum.
These lamps, as they age, develop less lumens, show an increase in voltage and a lowering of their color rendering index (CRI).
Further, it has been found desirable to provide these metal halide arc discharge lamps with a shroud which comprises a generally cylindrical, light-transmissive member, such as quartz, that is able to withstand high operating temperatures. The arc tube and the shroud are coaxially mounted within the lamp envelope with the arc tube located within the shroud. Preferably, the shroud is a tube that is open at both ends; however, in other cases, the shroud is open on one end and has a closed, domed configuration at the other end. Shrouds for metal halide arc discharge lamps are disclosed in U.S. Pat. Nos. 4,499,396; 4,580,989; and 4,281,274, as well as the above-mentioned 5,323,091.
The quartz shrouds employed by the prior art lamps are expensive because of their size as characterized by their wall thickness which is 3 mm, this size having been determined to be necessary because of the mass of the arc tubes which are constructed from quartz having a wall thickness of 1 mm.
Shrouds of hard glass having a lesser thickness are disclosed in co-pending application Ser. No. 08/033,525, filed Mar. 3 1993 and assigned to the assignee of the instant application.